1. Field of the Invention
The present invention relates to a position detector that employs a scale unit having a linear encoder and so forth. The present invention relates more particularly to a position detector having a function capable of detecting irregularities, such as a failure in the encoder and a break in a cable, and cable for use in the detector.
2. Description of the Related Art
In a positioning device for a machine tool and semiconductor manufacturing equipment, output signals from a scale unit having a linear encoder can typically be employed as position feedback signals for a controller. The position signals from the linear encoder to the controller may often be transmitted in the form of differential signals with A/B/Z phases on the basis of RS422A. The conventional controller has the following functions for detecting irregularities of the outputs from the linear encoder;
(1) A function for detecting a break and poor connection in a cable between the output of the encoder and the input of the controller; and PA1 (2) A function for detecting an irregularity in the output value from the encoder.
With respect to (1), detecting a high-impedance state of the controller input can perform the irregularity detection. As for (2), detecting if a deviation between position instruction information and position feedback information from the scale unit (encoder) is an abnormal value can perform the irregularity detection. Some controllers may not have the function (2). In those cases, an irregularity detecting circuit is provided inside the scale unit for diagnostic checks on itself. The circuit varies its output to a high-impedance state when an irregularity occurs so that the controller can detect the irregularity of the encoder output.
The above irregularity detection method for the conventional position detector, however, has such a disadvantage that the controller can not determine whether the high-impedance state detected is caused from a break in the signal cable or an output irregularity of the encoder in the scale unit. This makes it difficult to perform a fast procedure at the time of maintenance.
A system has also been known, which has an indication function for indicating an irregularity state in a pulse unit that is arranged separately from the scale unit. The pulse unit for use in this system converts analog signals supplied from an encoder, such as bi-phase pseudo-sinusoidal wave signals, into bi-phase rectangular wave signals with a required resolution at an interpolation circuit. The pulse unit outputs position pulse signals with A/B/Z phases to the controller through a driver corresponding to RS422. In addition, the pulse unit detects if any irregularities occur in the level of the encoder signal and the state of the interpolation circuit. When an irregularity occurs, the pulse unit varies its output for the controller to a high=impedance state and at the same time discriminates the irregular phenomenon to indicate it on a display.
The system using such the pulse unit is excellent in maintainability, however, it also has a disadvantage that the pulse unit prevents the space and cost from reducing because it must be arrange between the scale unit and the controller but is an separate unit from them. There is another system that contains the above-described function of the pulse unit inside a scale unit for the purpose of reducing the space and cost. In this case, the scale unit may often be mounted on a periphery of a movable element of the positioning device. Therefore, it is difficult to confirm an indicator by human eyes and thus such the indicator can not be employed. As a result, an irregular state can not be identified and a rapid procedure at the time of maintenance can be hardly achieved.